1. Field of the Invention
This invention relates to an optical coordinate input device for detecting position information selected and, more particularly, to an improvement of its detection performance.
2. Description of the Prior Art
In general, the optical coordinate input device is disposed in front of a picture screen, for example, of a CRT display and functions in such a manner that as a light signal is intercepted by selecting a desired position by means of a finger, for example, in correspondence with the information displayed on the screen, information concerning that position can be provided. The optical coordinate input device of the above type is in demand increasingly in the field of input devices, for example, personal computers. Further, the optical coordinate input device of this type is degraded little mechanically because a coordinate is detected by means of the light signal; thus, its capability as an input device is expected to increase greatly in the future.
However, the optical coordinate input device of the above type is composed of a number of light emitting and light receiving elements, so that variations in the elements influence largely its optical conversion characteristic. In addition, the light receiving element is susceptible to disturbance light such as the sunlight, thereby simply resulting in malfunctions. Accordingly, it is strongly desired that the optical coordinate input device should not cause malfunctions even under such circumstances as above.
An example of the conventional optical coordinate input device will now be described with reference to FIGS. 11 and 12. FIG. 11 is a circuit configuration diagram of the conventional optical coordinate input device, and FIG. 12 is a waveform diagram of signals at portions of a waveform shaping circuit shown in FIG. 11.
In FIG. 11, reference numeral 1 indicates a drive circuit, 3 is an amplifier circuit, 4 is a waveform shaping circuit, 5 is a CPU, 6 and 7 are first and second switching circuits.
The drive circuit 1 operates on the basis of a drive signal a given from the CPU 5 which is a microprocessor. Into the base of a transistor Q.sub.3 of the drive circuit 1 the drive signal a is input, to its collector a supply voltage is applied from a power terminal Vcc, and to its emitter the first switching circuit 6 is connected which supplies a current to light emitting diodes L.sub.1 -L.sub.n functioning as the light emitting elements.
Switching elements SL.sub.1 -SL.sub.n of the first switching circuit 6 are switched successively on the basis of a switching signal b given from the CPU 5 so that only one circuit is closed at a time, starting from the switching element SL.sub.1. To one ends of these switching elements SL.sub.1 -SL.sub.n the emitter of the transistor Q.sub.3 is connected, and to the other ends the anodes of the light emitting diodes L.sub.1 -L.sub.n are connected respectively.
The cathodes of the light emitting diodes L.sub.1 -L.sub.n are grounded. Among the light emitting diodes L.sub.1 -L.sub.n one light emitting diode selected by the switching signal b is rendered conductive by the drive circuit 1 so as to emit light a given number of times, and these light emitting diodes L.sub.1 -L.sub.n provide light pulses successively in this order. They are arranged in two straight lines so that the optical axes of the light emitting diodes L.sub.1 -L.sub.m intersect orthogonally those of the light emitting diodes L.sub.(m+1) -L.sub.n.
Phototransistors PT.sub.1 -PT.sub.m and PT.sub.(m+1) -PT.sub.n functioning as the light receiving elements for receiving the light pulse signals given from the light emitting diodes L.sub.1 -L.sub.n are arranged straight opposite to the light emitting diodes L.sub.1 -L.sub.m and L.sub.(m+1) -L.sub.n, respectively. The emitters of the phototransistors PT.sub.1 -PT.sub.n are grounded, and their collectors are connected to one-side terminals of the corresponding switching elements S.sub.1 -S.sub.n of the second switching circuit 7. The other-side terminals of the switching elements S.sub.1 -S.sub.n are connected to a condenser C.sub.1 and resistor R.sub.4 of the waveform shaping circuit 4 so that the supply voltage is applied thereto from the power terminal Vcc through resistors R.sub.1 and R.sub.4. These switching elements S.sub.1 -S.sub.n close one circuit at a time successively, starting from the switching element S.sub.1 on the basis of the switching signal b supplied from the CPU 5. The switching action of the second switching circuit 7 is identical in timing to the switching signal b applied to the first switching circuit 6, so that the light emitting diodes L.sub.1 -L.sub.n and corresponding, opposed phototransistors PT.sub.1 -PT.sub.n are concurrently put into operation.
When the phototransistors PT.sub.1 -PT.sub.n receive the light signals given from the light emitting diodes L.sub.1 -L.sub.n, they increase or decrease their currents on the basis of that light signals and by the action of the resistors R.sub.1 and R.sub.4, such a pulse voltage waveform as indicated by V.sub.PT in FIG. 12 is generated.
This pulse voltage waveform V.sub.PT is applied through the the coupling condenser C.sub.1 to the base of the transistor Q.sub.1 in the form of a pulse voltage waveform like that indicated by V.sub.B in FIG. 12. To the base of the transistor Q.sub.1 a d.c. bias voltage is also applied through a resistor R.sub.5 from the power terminal Vcc; thus, the pulse voltage waveform V.sub.B takes the composed voltage value of the base-emitter forward Zener voltage V.sub.BE (V.sub.BE .apprxeq.+0.6 V) of the transistor Q.sub.1 caused by the d.c. bias voltage and of the pulse voltage applied through C.sub.1 from the switching circuit 7. As the pulse voltage waveform V.sub.B is applied to the base of the transistor Q.sub.1 and only when it takes a value exceeding the base-emitter forward Zener voltage V.sub.BE, a base current like that indicated by I.sub.B in FIG. 12 flows. On the basis of the waveform of this base current I.sub.B a collector current flows in the circuit connected from the power terminal Vcc to the collector of the transistor Q.sub.1 through the resistors R.sub.1 and R.sub.2, and a signal of voltage fluctuation caused by a change of the above current is provided to the amplifier circuit 3. To cut away a pulse fluctuation of the voltage at the point where the resistors R.sub.1, R.sub.2, and R.sub.4 are connected together, a condenser C.sub.2 is connected to that point for bypassing the pulse voltage fluctuation.
Then, on the basis of the output signal of the waveform shaping circuit 4 given through the amplifier circuit 3, the CPU 5 provides a coordinate signal corresponding to the state of light signals received by the phototransistors PT.sub.1 -PT.sub.m and PT.sub.(m+1) -PT.sub.n. This output represents the position of the phototransistors which do not receive the light signals due to their interception by the finger, for example, so that the position information on the screen can be obtained therefrom.
However, the conventional optical coordinate input device of the foregoing configuration has the following problems:
Because the pulse voltage waveform V.sub.PT is generated by switching a number of light emitting and light receiving elements having variations in photoelectric conversion characteristics by the use of the first and second switching circuits 6 and 7, and because these light emitting and light receiving elements are disposed in front of a picture display device and configured so as to be easily susceptible to sunlight or disturbance light of non-uniform strength given from other lighting equipment and the like, the pulse voltage waveform V.sub.PT obtained from each light receiving element varies largely from element to element, and such a variation tends to fall outside the variation-absorbable range of a clamp circuit for absorbing variations which utilizes the condenser C.sub.1 and the base-emitter forward Zener voltage characteristic V.sub.BE of the transistor Q.sub.1.
In addition, malfunctions occur in detecting coordinates due to switching noises arising upon switchover of the elements.
According to the prior art, in order to prevent occurrence of malfunctions, an optical filter are attached in front of the light emitting and light receiving elements so as to reduce an influence of disturbance light; but, on the other hand, this optical filter lowered the photoelectric conversion efficiency of the light emitting and light receiving elements, thus degrading the light detector performance.
To reduce such a baneful influence, it was required to select and use light emitting and light receiving elements of a superior photoelectric conversion efficiency; thus, the optical coordinate input device needing a number of such elements could not be inexpensively mass-produced.
In addition, the light emitting and light receiving elements, such as phototransistors PT.sub.1 -PT.sub.n and LEDs for providing the light signal, have variations in their light characteristic. Further, the phototransistors PT.sub.1 -PT.sub.n are influenced by a change in strength of light, for example, of the CRT display or disturbance light. As a result, the collector voltage V.sub.PT of each phototransistor varies. Therefore, as indicated by V.sub.PT in FIG. 12, if the collector voltage V.sub.PT of the transistor PT.sub.2 lowers remarkably due to disturbance light and the like when that phototransistor PT.sub.2 has received the light signal, the base voltage V.sub.B of the transistor Q.sub.1 also lowers as indicated by V.sub.B in FIG. 12; thus, the transistor Q.sub.1 tends to be kept in the OFF state during the scanning period of the phototransistor PT.sub.2. If it happens, as indicated by I.sub.B in FIG. 12, the coordinate signal is indicated as being in a blocked state; thus, despite the fact that the phototransistor PT.sub.2 has received the light signal, the personal computer or the like decides erroneously that the coordinate has been entered. On the other hand, because the voltage is applied through the resistor R.sub.5 to the base of the transistor Q.sub.1, a charging current Ic flows into the coupling condenser C.sub.1 as shown in FIG. 11. Therefore, when the collector voltage V.sub.PT of the phototransistor lowers, as indicated by PT.sub.2 or PT.sub.(n-1) of V.sub.B in FIG. 12, the falling level of the base voltage V.sub.B of the transistor Q.sub.1 is raised. However, because the base voltage V.sub.B rises simply gradually in accordance with the time constant of the resistor R.sub.5 and coupling condenser C.sub.1, when the collector voltage V.sub.PT of the phototransistor lowers remarkably, the coordinate signal keeps its high state as described hereinabove. Then, if, for example, as indicated by V.sub.PT in FIG. 12, the light signal is received at the moment the collector voltage V.sub.PT of the phototransistor PT.sub.3 has risen a little, the base current I.sub.B of a small level tends to flow into the transistor Q.sub.1 or distortion tends to appear in the base current I.sub.B curing its rising period (see the waveform of I.sub.B in FIG. 12); thus, it becomes difficult to judge whether or not the coordinate signal provided implies a pulse waveform. Therefore, in this case also, there is the fear that the personal computer or the like registers an erroneous detection.
Further, at the time the switches S.sub.1 -S.sub.n of the switching circuit 7 have been switched over by the use of a synchronization pulse, due to switching noises the collector voltage V.sub.PT each of the phototransistors PT.sub.1 -PT.sub.n falls momentarily, and the base voltage V.sub.B of the transistor Q.sub.1 also falls momentarily. As a result, at the time of switchover of the switches S.sub.1 -S.sub.n, the base current I.sub.B of the transistor Q.sub.1 falls or is cut off momentarily, and the noise signal is added to the coordinate signal. Accordingly, there is the fear that the personal computer or the like decides this noise signal be the coordinate signal, and the foregoing becomes a cause of erroneous detection.
As described hereinabove, the base-emitter junction of the transistor Q.sub.1 functions as the diode and forms the clamp circuit in conjunction with the coupling condenser C.sub.1 and resistor R.sub.5, and thus performs compensation through its clamp action so as to make the base voltage V.sub.B assume uniformly a given level when that voltage V.sub.B lowers. However, if the base voltage V.sub.B changes, as described above, due to variations in the optical characteristic of the light emitting and light receiving elements and/or an influence of disturbance light and switching noises, the base voltage V.sub.B cannot be compensated for up to a given level even by the clamp action.
By setting the resistance of the resistor R.sub.5 to be small the clamp action can achieve an enhanced extent of compensation; but, with a small resistance of the resistor R.sub.5, the amount of excess storage carriers accumulated within the base of the transistor Q.sub.1 increases and the impedance on the base side of the transistor Q.sub.1 lowers. Therefore, with respect to the falling and rising of the collector voltage V.sub.PT of the phototransistor, the falling and rising of the base voltage V.sub.B of the transistor Q.sub.1 lag, and the waveform of the base voltage V.sub.B is distorted. Accordingly, in case the light emitting and light receiving elements are scanned at a high speed, this also becomes a cause of erroneous detection.